The invention relates to an optical modulation arrangement for modulating a beam of optical radiation generated by a laser source. The arrangement includes an optical modulator for modulating the amplitude of the beam by means of an electrical signal, followed by a laser amplifier for amplifying the modulated beam.
Laser beams having a high energy density capable of melting or vaporizing substances, are often employed in industry for performing machining operations on workpieces, such as for example milling, cutting, or drilling. Consequently, many forms of high power lasers have been developed for this purpose many of which have to be employed in pulsed operation using for example Q-switching. These types of lasers require a considerable time interval between pulses in order to repopulate the lasing energy level and to keep the average dissipation down to a level below that at which damage to the material of the laser occurs. In some cases this interval is as great as 2 seconds.
These remarks mainly apply to solid state lasers. In the case of gas lasers, the highest power has been obtained from the CO.sub.2 laser which is capable of continuous operation at 10 KW. Certain industrial operations, however, require a highly accurate and continuous control of the energy of the work beam at high modulation frequencies, for example when a fine and intricate image pattern has to be produced directly on a metal die or plate for embossing or printing.
IEEE Spectrum, Vol. 9 (April 1972), pages 62-72, mentions a modulated laser beam system developed at the Bell Telephone Laboratories, for generating a primary pattern for an IC mask on a photographic plate. The system uses a low powered argon laser followed by an acousto-optic modulator and a mechanical scanning arrangement for which beam modulation frequencies in excess of 1 MHz were employed. Since this was a photographic process only low power was required. However, when it is desired to apply a pattern directly to metal or other substance by thermal attrition, the problem arises that suitable optical modulators have very restricted power handling capabilities and it becomes necessary to employ a low-power source-laser to feed the optical modulator, and then to amplify the modulated radiation beam by means of a laser amplifier to provide the desired power level for the work beam.
The main forms of optical modulator are electro-optic, magneto-optic and acousto-optic. Electro-optic modulators are electrically difficult to drive and to match because of the high drive voltage and large associated capacity, and are difficult to manufacture with a sufficiently good optical quality. However, their main disadvantage for the applications mentioned is that it can be difficult to provide a satisfactorily low beam extinction level. This latter difficulty can also occur with magneto-optic modulators. By contrast, the acousto-optic modulator, in which the laser beam is refractively Bragg diffracted by an amplitude modulated acoustic beam propagating in an acoustic medium, can reliably provide complete extinction of the modulated output beam provided that the latter is formed by the diffracted beam.
However, since the diffracted beam is generated by a refraction structure corresponding to the local stress pattern caused in the acoustic medium by the propagating acoustic waves, this structure will also propagate with the velocity of the acoustic wave and will cause the optical frequency of the diffracted modulated beam to change by an amount equal to the frequency of the acoustic wave which may be, typically, 60 to 100 MHz.
The type of laser that would normally be employed as a laser amplifier for a potentialy continuous high output power would be a form of low pressure CO.sub.2 laser, working for example at a CO.sub.2 gas pressure of about 1 torr, preferably with continuous renewal of CO.sub.2, and in the amplified beam arrangement the source laser would also be a low pressure CO.sub.2 laser. However, if an acousto-optic modulator were employed as the optical modulator, the consequent frequency conversion of the diffracted modulated beam discussed above, would shift the frequency of the modulated beam outside the narrow amplification bandwidth of the subsequent CO.sub.2 laser amplifier.